You must have installed the PopClip trial version from my website at some point. Just quit and delete that and install PopClip from the Mac App Store.
One gotcha is that you might have more than one copy of PopClip on your Mac. Maybe you installed it from Mac App Store to /Applications but also downloaded it from the website to /Downloads at some point. Clear any and all copies of PopClip out, and install just the one copy from App Store.
I solved the problem by quitting and restarting and just clicking “open” in the Mac App Store. But really, at no point did I install a new one (let alone a trial version). The version I use daily is the one from the Store. Surely it is strange that the problem occurred immediately after I ran this command?
The Mac App Store edition doesn’t contain the code to show the trial messages, so if you saw “trial expired”, you must have had a standalone edition running. Maybe you tried a beta version or some bug fix I posted at some point. Anyway, glad it’s working now.